Ink jet printing is accomplished by ejecting ink from a nozzle toward paper or other print medium. The ink is driven from the nozzles toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven from a nozzle toward a medium with an electrostatic field. Another ink jet printing procedure, known as squeeze tube, employs a piezo-electric element in the ink nozzle. Electrically-caused distortions of the piezo-electric element pump the ink through the nozzle and toward the print medium. In still another ink jet printing procedure, known as thermal or bubble ink jet printing, the ink is driven from a nozzle and toward the print medium by the formation of an expanding vapor-phase bubble in the nozzle. These various printing methods are described in Output Hard Copy Devices, edited by Durbeck and Sherr, Academic Press 1988, see particularly chapter 13 entitled Ink Jet Printing.
Hot melt ink compositions for use in ink jet printers are generally crystalline or semicrystalline and generally comprise a polymeric binder, an organic solvent and colorant. The compositions are solid at room temperature to avoid evaporation or spillage during periods of nonprinting. For hot melt ink compositions for thermal ink jet printers, the organic solvent functions as a bubble former which vaporizes in the heating chamber to drive the ink out of the nozzle and towards the print medium.
During use, the hot melt ink is heated so that it melts and the melted ink is propelled through the nozzle onto the recording paper. The rapidly cooling ink then solidifies on the surface of the paper. Unfortunately, clue in large part to the crystalline nature of the ink, the ink dot can be smeared on the recording paper by friction or pressure. In order to mitigate the problem of smearing, the molecular weight of the polymeric binder was increased to form a more cohesive solid. Unfortunately, the increase in molecular weight of the polymeric binder caused a corresponding increase in the viscosity of the melted ink which exacerbated drop misdirection and clogging problems during the jetting process. Further, the crystalline nature of the ink composition causes specular reflections from the crystal faces resulting in lower optical density. In order to avoid the problems associated with polymeric binders Tochihara et al., in U.S. Pat. No. 5,131,949 issued Jul. 21, 1992, discloses the use of ink formulations comprising amides or bisamide as the binder. However, Tochihara teaches that these compounds are crystalline.
Therefore, there still is a need in the art for an ink composition for ink jet printers which has low viscosity in the liquid state and has good smear resistance and optical density.
It is therefore an object of the present invention to provide an improved ink composition for ink jet printers.
Other objects and advantages will become apparent from the following disclosure.